Charge transfer circuits can be used to actuate electromagnetic actuators or in the area of sensor readouts or for any general charge nulling application. For example in the area of sensor readouts, a photosensitive diode can charge a capacitor and, in order to know how much light was received by the diode it can be desired to measure the charge of the capacitor by counting how many transfers of a predetermined charge are necessary to empty the capacitor.
Sensor readout schemes such as described in “Design Approaches for Digitally Dominated Active Pixel Sensors: Leveraging Moore's Law Scaling in Focal Plane Readout Design” by Tyrrell, et al, in Proc. Of SPIE Vol. 6900, (2008) use charge transfer or charge subtracting circuits.
Other sensor readout schemes, such as described in “Flexible Readout and Integration Sensor (FRIS): A Bio-Inspired, System-On-Chip, Event-Based Readout Architecture” by Lin, et al in Proc. of SPIE Vol. 8353, (2012) use a Sigma-Delta ADC that requires a 1-bit DAC, where a switched capacitor 1-bit DAC can be described as a charge transfer circuit.
FIG. 1 illustrates a known switched capacitor 1-bit DAC 10 having a capacitor 12 that outputs a transferred charge of: Qsc=C×(VDD−VTHP−Vsc). It is noted that the transferred charge depends on the threshold voltage VTHP of the output transistor M5, where such threshold voltage varies with flicker and RTS noise. As the size of the sensor array grows, the size of the transistors decreases and the effects of flicker and random noise makes difficult to pick signals of interest.
Next generation imagers, such as infrared imagers, may require high resolution digital read-out circuits to achieve the required fidelity in imaging. The precision and sensitivity of sensor readouts depend greatly on minimizing the effect of noise on the charge that is effectively transferred by any charge transfer circuit used for the sensor readout. The inventor has noted that the charge that is effectively transferred by known charge transfer circuits suffers from errors caused by flicker noise or random telegraph noise, in particular when the charge transfer circuit is a MOS circuit.
There exists a need for a charge transfer circuit that can transfer a precise charge without being sensitive to flicker and RTS noise.